Acrylic acid is, for example, produced in the following process. That is, first of all, propane or propylene as the starting material is subjected to vapor phase catalytic oxidation, and the resulting oxidation reaction mixture is absorbed in an absorbing solvent such as water, thereby recovering a solution of acrylic acid. This solution contains various impurities formed as by-products during the vapor phase catalytic oxidation, such as acetic acid, maleic acid, acrolein, furfural, benzaldehyde, and acetone, other than acrylic acid. In a preliminary purification step, the absorbing solvent and a part of the by-products are removed from the resulting acrylic acid solution to obtain crude acrylic acid consisting essentially of acrylic acid and its dimer and other heavy components. Subsequently, the crude acrylic acid is purified in a vacuum distillation column, etc. in a purification step to recover high-purity acrylic acid.
In the production process of an acrylic acid ester, first of all, acrylic acid is reacted with an alcohol in the presence of an acid catalyst, and the resulting reaction mixture is then purified in a vacuum distillation column to recover a high-purity acrylic acid ester.
In the production process of methacrylic acid or a methacrylic acid ester, methacrylic acid or a methacrylic acid ester is, for example, produced in the same process as described above using isobutylene as the starting material.
Hitherto, even though a problem of plugging does not occur in a relatively small-sized distillation column as in experimental equipment, serious plugging may possibly occur in a large-sized distillation column as in commercial equipment. It is considered that this is caused due to the matter that non-uniformity within the column increases with an increase in the instrumental size. For example, even when the degree of horizontality of a distillation column, i.e., the gradient per unit length, is identical, the difference of elevation increases in proportion to the column diameter of the distillation column.
In the case where it is intended to compensate the increase in non-uniformity with an increase in the instrumental size by precision, execution becomes difficult as the instrumental size increases.
In operating a vacuum distillation column of readily polymerizable compounds, a pressure control valve is plugged due to polymerization in a condensate, resulting in a problem such that it becomes impossible to control the degree of vacuum in the distillation column within a fixed range.
In a tray of a distillation apparatus, in the case where an acrylic acid-containing liquid is passed through pores provided on a weir-free perforated tray and flown down (see JP-A-2000-300903, etc.), a part of the liquid travels on the inner wall of the pore and goes around into the back face of the perforated tray, and the resulting liquid becomes one of the factors to form a polymer, causing a matter that long-term continuous production of (meth)acrylic acids cannot be carried out.
In continuously operating a distillation column, it is the most problematic that a differential pressure within the distillation column, namely a difference between the column bottom pressure and the column top pressure increases. The elevation of the differential pressure within the column is caused by the matter that the pore size on the perforated tray is reduced by the polymer, whereby both a liquid and a gas become to hardly flow. When accumulation of the polymer advances, some pores are completely plugged. Ultimately, both the liquid and the gas cannot sufficiently flow, whereby the operation of the distillation column is unavoidably stopped.
Also, the production equipment of (meth)acrylic acid, etc. includes column equipment such as a distillation column of crude (meth)acrylic acid, etc. or a decomposition reaction column of decomposing high-boiling materials to recover (meth)acrylic acid, etc. In the column equipment, an introducing tubular member in which a part of the column bottom liquid is introduced into a reboiler and heated, and then returned to the column bottom is provided connecting to the bottom face of the lower end of the column.
According to the conventional connection structure of the tubular member for introducing a column bottom liquid to the reboiler, polymers, a polymerization inhibitor and others contained in the column bottom liquid are liable to incorporate into the introducing tubular member. And the polymers and polymerization inhibitor accumulate in the reboiler, thereby possibly causing plugging. Incidentally, the polymerization inhibitor is added in the production step of readily polymerizable compounds. The polymers include ones carried from the equipment in the upstream side and ones formed within the column equipment.
With respect to a countermeasure against the plugging, in the case where a pump for forced circulation is provided in the introducing tubular member, it is known to provide a strainer in the introducing tubular member, thereby separating the polymers and polymerization inhibitor by filtration. However, if the opening of the strainer is made large, the separation by filtration becomes insufficient so that an effect for preventing plugging of the reboiler is insufficient. Conversely, if the opening of the strainer is made small, clogging of the strainer frequently occurs so that the works of the column equipment likely become instable. Incidentally, in a spontaneous circulation type reboiler, such a strainer cannot be placed, and a measure for preventing inflow of the polymers and polymerization inhibitor into the reboiler is not employed.
For the sake of preventing polymerization of a readily polymerizable compound within an instrument, the following countermeasures have hitherto been employed, for example. That is, a chemical countermeasure is to delay or retard polymerization reaction of acrylic acid with an initiator as a radical, in which a polymerization inhibitor of capturing the radical is, for example, added to the absorbing solvent or in the preliminary purification step or purification step, or the operating temperature is lowered for the purpose of delaying the radical formation. As the polymerization inhibitor of acrylic acid, gaseous materials such as oxygen and a variety of organic or inorganic compounds are known. Concretely, p-hydroquinone, p-methoxyphenol, phenothiazine, and others are known. Usually, in many cases, oxygen and an organic or inorganic compound are used jointly. For the sake of lowering the operating temperature, an operation pressure of the distillation operation is lowered.
In the case where an organic or inorganic compound is used as the polymerization inhibitor, such a compound is in general used as a solution thereof. For example, JP-A-10-17524 describes that a solution prepared by dissolving a copper compound such as copper carbonate and copper hydroxide and hydroquinone as polymerization inhibitors in an acetic acid aqueous solution obtained in distilling off water from a (meth)acrylic acid solution by dehydration distillation in a preliminary purification step is used. Though the polymerization inhibitor solution obtained by such a method has excellent performance, there is involved a problem such that the acetic acid component to be removed in the preliminary purification step is carried into the system, thereby increasing a purification load in the preliminary purification step.
A physical or mechanical countermeasure is to prevent the generation of polymerization due to local deviations in formulation, flow rate, residence time, temperature, and others. For example, it is achieved by reducing the residence of the liquid or gas by changing the instrumental shape or by lowering the roughness of the instrument surface.
However, as described previously, the serious plugging problem in a large-sized distillation column as in the commercial equipment has not been solved yet.
A problem of the invention is to solve the serious plugging problem in a large-sized distillation column as in the commercial equipment and to provide an apparatus and a method for distilling and purifying a readily polymerizable compound stably over a long period of time. In more detail, the following points are enumerated.
A first problem of the invention is to provide a process of producing a purified readily polymerizable compound, wherein in distilling and purifying a readily polymerizable compound, an improvement is made such that plugging of a pressure control valve is prevented and the readily polymerizable compound carried in an exhaust gas of a vacuum generator can be easily recovered.
A second problem of the invention is to enable one to control inflow of polymers or polymerization inhibitor into a reboiler by a simple mechanism, thereby making it possible to continuously operating column equipment stably over a long period of time.
A third problem of the invention is to provide a perforated tray capable of preventing occurrence of polymerization of a (meth)acrylic acid when it goes around into the back face of the tray and a process of producing a (meth)acrylic acid using the same.
A fourth problem of the invention is to provide a method of distilling (meth)acrylic acid in which by preventing plugging of an opening of a weir-free perforated tray by polymer and preventing elevation of a differential pressure within a column, a (meth)acrylic acid solution can be distilled stably over a long period of time.
A fifth problem of the invention is to provide a process of producing (meth)acrylic acid in which a purification load is reduced in a preliminary purification step.